Hemichordata
name = Hemichordata image = Acorn worm.PNG image_width = 150px image_caption = a species of the genus Saccoglossus
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Hemichordata is a phylum of worm-shaped marine deuterostome animals, generally considered the sister group of the chordates. They date back to the Lower or Middle Cambrian and include an important class of fossils called graptolites, most of which became extinct in the Carboniferous. They seem to have a primitive form of notochord, but this is most likely the result of convergent evolution. A hollow neural tube exists among some species (at least in early life), probably a primitive trait they share with the common ancestor of chordata and the rest of the deuterostomes. The musculature in their gut is very poorly developed, and food is mostly transported through it by using the cilia that covers its inside. They are tradtionally considered as a sister group to the echinoderms.
Hemichordata are divided into two classes: the Enteropneusta, commonly called acorn worms, and the Pterobranchia, which may include the graptolites. A third class, Planctosphaeroidea, is proposed based on a single species known only from larvae. The phylum contains about 100 living species. The exact taxonomic position of hemichordata and whether the group is monophyletic is currently under debate. One of the suggestions are that the pterobranchs are more basal deuterostomes, while the enteropneusts are an early offshoot of the lineage who are leading to Chordata.
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